In recent years, there has been developed a power generator which can generate electric power by utilizing variation of magnetic permeability of a magnetostrictive rod formed of a magnetostrictive material (for example, see patent document 1).
For example, this power generator includes a pair of magnetostrictive rods arranged side by side, two connecting yokes for respectively connecting one end portions and the other end portions of the pair of magnetostrictive rods with each other, coils arranged so as to respectively surround the magnetostrictive rods, two permanent magnets respectively arranged on the two connecting yokes to apply a bias magnetic field to the magnetostrictive rods and a back yoke. The pair of magnetostrictive rods serve as beams facing each other. When external force is applied to one of the connecting yokes in a direction perpendicular to each axial direction of the pair of the magnetostrictive rods, one of the magnetostrictive rods is deformed so as to be expanded and the other one of the magnetostrictive rods is deformed so as to be contracted. At this time, magnetic permeability of each magnetostrictive rod 2 varies. This variation of the magnetic permeability of each magnetostrictive rod 2 leads to variation of density of lines of magnetic force (magnetic flux density) passing through the magnetostrictive rods (that is density of the lines of magnetic force passing through the coils), thereby generating a voltage in the coils.
Generally, the magnetostrictive rod as described above has characteristics that a variation amount (decreasing amount) of the magnetic flux density (magnetic permeability) caused by generation of compressive stress is large while a variation amount (increasing amount) of the magnetic flux density (magnetic permeability) caused by generation of tensile stress is small. Thus, it is difficult to sufficiently vary the magnetic flux density of each of the magnetostrictive rods even if the external power is applied to the power generator to alternately generate the tensile stress and the compressive stress in each of the magnetostrictive rods because the variation amount of the magnetic flux density caused by the tensile stress is small.
Further, the variation amount of the magnetic flux density of each of the magnetostrictive rods is affected by an intensity of the bias magnetic field applied to the magnetostrictive rods. Generally, the variation amount of the magnetic flux density decreases as the intensity of the applied bias magnetic field increases.
Thus, in the power generator disclosed in the patent document 1, if the intensity of the bias magnetic field applied to the magnetostrictive rods is large, it is impossible to sufficiently increase the variation amount of the magnetic flux density of each of the magnetostrictive rods (specifically, it is required to increase the variation amount of the magnetic flux density to about 1 T) without significantly increasing the external force applied to the magnetostrictive rods to sufficiently increase magnitudes of the tensile stress and the compressive stress caused in each of the magnetostrictive rods. Thus, if the intensity of the bias magnetic field applied to the magnetostrictive rods is large, it is difficult for the power generator disclosed in the patent document 1 to efficiently generate the electric power.